In production of substrates each with a semiconductor integrated circuit thereon, irregularities or steps are generated when a circuit is formed on a silicon wafer with metal such as copper, and therefore metal portions of the circuit are preferentially removed by polishing to remove the irregularities or steps in a surface. Furthermore, irregularities are generated when aluminum wiring is provided on a silicon wafer and an oxide film of silica or the like as an insulating film is provided, and the irregularities are averaged by polishing the oxide film. In the process of polishing a substrate as described above, it is generally required that the polished surface does not include any step nor irregularity and is flat and smooth without any micro blemish or the like, and that the polishing speed is high.
Recently, in association with the needs for size reduction and higher performance of electric and electronic products, a degree of integrated in semiconductor devices has been becoming increasingly higher, but, for instance, when there remain impurities such as Na or K in a separating layer of a transistor, the desired performance may not be realized or some failures may occur. Especially, when Na is deposited on a surface of a polished semiconductor substrate or of an oxide film, Na having high dispersibility may be trapped in defects of the oxide film or the like. In this case, even when a circuit is formed on the semiconductor substrate, sometimes an insulation failure or a short circuit may occur, and furthermore the dielectric constant may drop. Because of the tendency, under some conditions for use of the semiconductor substrate, or when the semiconductor substrate is used for a long time, the failures as described above may occur, so that there is a strong need for development of a polishing material containing little impurities such as Na or K.
As particles for polishing, such materials as a silica sol, fumed silica, and fumed alumina are generally used.
A polishing material used in the CMP technique generally includes spherical particles for polishing with the average diameter of around 200 nm made of a metal oxide such as silica or alumina, an oxidizing reagent for raising an operational speed in polishing metal material for wiring or circuits, and additive such as an organic acid, and solvent such as deionized water. Since there are steps (irregularities) caused by a trench pattern for wiring formed on the base insulating film on a surface of a polished material, the surface is polished down to the common plane by mainly removing the convex portions to provide a flat polished surface. With the spherical particles as described above, however, when a portion above the common plane is polished, the metal for a circuit within a wiring trench under concave sections may disadvantageously be polished down to a level under the common plane, and the problem is generally referred to as dishing. When the dishing (excessive polishing) as described above occurs, there occur such problems as that a thickness of the wiring diminishes and the wiring resistance becomes larger, or that the flatness of an insulating film formed on the polished surface becomes lower, so that there is the need for suppressing the dishing.
When a polishing material containing a group of nodular particles is used for polishing a substrate having the irregularities as described above, concave portions on a surface of the substrate are prevented from being polished until upper end faces of the convex portions are polished to the same level as bottom faces of the concave portions. After the upper end faces of the convex portions are polished down to the same level as the bottom faces of the concave portions, both the convex portions and the concave portions can be polished at the same polishing speed, the dishing (excessive polishing) never occurs, and the polished surface has no irregularities and is excellent in the flatness as well known. Since the dishing never occurs in a polishing process performed, for instance, when forming a semiconductor integrated circuit, increase of a circuit resistance in the obtained integrated circuit does not occur, and the polished surface is excellent in the flatness, so that a laminated integrated circuit can efficiently be formed.
Applications of the polishing material containing the nodular particles as described above expectedly include, but not limited to mirror machining of aluminum wiring on a aluminum disk (aluminum or a metal skin over the substrate) or on a semiconductor multilayered wiring substrate, a glass substrate for an optical disk or a magnetic disk, a glass substrate for a liquid crystal display, a glass substrate for a photo mask, and other glass materials.
As a method of producing a silica sol containing nodular particles, for instance, Japanese Patent Laid-Open Publication No. HEI 4-187512 (Patent document 1) discloses the method in which a silicic acid solution is added to an aqueous solution of an alkali metal silicate with the concentration in the range from 0.05 to 5.0 weight % when calculated as SiO2 so that the SiO2/M2O (molar ratio, wherein M denotes an alkali metal or quarternary ammonium) in the mixture solution is in the range from 30 to 60, and then one or more metal compounds selected from the group consisting of Ca, Mg, Al, In, Ti, Zr, Sn, Si, Sb, Fe, Cu and rare earth metals are added to the mixture solution (before or during the process of adding the silicic acid solution), the resultant mixture solution is maintained at a temperature of 60° C. or more for a certain period of time, and furthermore a silicic acid solution is added to the mixture solution so that SiO2/M2O (molar ratio) is in the range from 60 to 100 to obtain a sol in which nodular silica particles are substantially dispersed.
Japanese Patent Laid-Open Publication No. HEI 7-118008 (Patent document 2) discloses a method of producing a silica sol of particles having a slender form in which water-soluble calcium salt, magnesium salt, or an aqueous solution of the compounds is added to a colloidal solution of active silicic acid, an alkali substance is added to the resultant aqueous solution, a portion of the obtained mixture is heated to 60° C. or more to obtain a heal liquid, a remaining portion of the mixture is used as a feed liquid, the feed liquid is added to the heal liquid, and the mixture solution is condensed to the SiO2 concentration in the range from 6 to 30 weight % by evaporating water during the adding process to obtain a silica sol having a slender form.
Japanese Patent Laid-Open Publication No. 2001-11433 (Patent document 3) discloses a method of a silica sol of particles having a beads-like form in which an aqueous solution containing either one or both of an water-soluble bivalent and tervalent metal salts is added to a colloidal solution of active silicic acid including the active silica by 0.5 to 10 weight % as calculated as SiO2 with the pH of 2 to 6 so that a molar ration of the metal compound (expressed as MO in a case of a bivalent metal salt and as M2O3 in a case of a tervalent metal salt, wherein M denotes a bivalent or tervalent metal atom, and O denotes a oxygen atom) against SiO2 in the colloidal solution of the active silicic acid is in the range from 1 to 10 weight %, the resultant mixture solution is well agitated to obtain a mixture solution (1), an acidic and spherical silica sol with the average diameter in the range from 10 to 120 nm and pH in the range from 2 to 6 is added to the mixture solution (1) so that a ratio (weight base) of a content (A) of silica originated from the acidic spherical silica sol against a content (B) of silica originated from the mixture solution is in the range from 5 to 100 and at the same time so that a total silica content (A+B) as calculated as the SiO2 concentration in a mixture solution (2) obtained by mixing the acidic spherical silica sol with the mixture solution (1) is in the range from 5 to 40 weight %, an alkali metal hydrate or the like is added to the mixture solution (2) to adjust the pH to 7 to 11, and a resultant mixture solution (3) is heated at a temperature in the range from 100 to 200° C. for 5 to 50 hours.
Japanese Patent Laid-Open Publication No. 2001-48520 (Patent document 4) discloses a method of silica sol in which alkyl silicate with the silica concentration in the range from 1 to 8 mol/liter, the acid concentration in the range from 0.0018 to 0.18 mol/liter, and water concentration in the range from 2 to 30 mol/liter is hydrolyzed with acid catalyst, the product is diluted with water so that the silica concentration is in the range from 0.2 to 1.5 mol/liter, and then alkali catalyst is added to the diluted solution to adjust the pH to 7 or more, the solution is heated to advance polymerization of the silicic acid to obtain a silica sol in which slender non-crystalline silica particles having the average diameter in the sideward direction in the range from 5 to 100 nm and that in the longitudinal direction in the range from 1.5 to 50 times of the sideward direction above when observed with an electron microscope are dispersed in the liquid dispersing element.
Japanese Patent Laid-Open Publication No. 2001-150334 (Patent document 5) discloses a method of producing a silica sol of distorted particles in which a salt of an alkali earth metal such as Ca, Mg, Ba is added in an acidic aqueous solution of active silicic acid with the SiO2 density in the range from 2 to 6 weight % obtained by subjecting an aqueous solution of an alkali metal silicate such as water glass to the processing for removing cation at the weight ration of the salt as calculated as an oxide thereon against SiO2 in the acidic silicic acid in the range from 100 to 1500 ppm, furthermore the alkali material is added to this solution so that the SiO2/M2O molar ratio (wherein M denotes an alkali metal atom, NH4, or a quarternary ammonium group) is in the range from 20 to 150 to obtain an initial heal liquid, an aqueous solution of active silicic acid obtained in the same way as that described above and having the SiO2 concentration in the range from 2 to 6 weight % and the SiO2/M2O molar ratio (M denotes an alkali metal atom) in the range from 20 to 150 is used as a charge liquid, and the charge liquid is added to the initial heal liquid at the temperature in the range from 60 to 150° C. at a rate expressed as a weight ratio of SiO2 in the charge liquid/SiO2 in the initial heal liquid per hour in the range from 0.05 to 1.0 evaporating (or not evaporating) water from the solution to obtain a sol of distorted particles.
Japanese Patent Laid-Open Publication No. HEI 8-279480 (Patent document 6) describes that, in the colloidal silica aqueous solution prepared by the method (1) in which an alkali aqueous solution of silicic acid is neutralized with a mineral acid, then an alkali substance is added to the neutralized aqueous solution, and the mixed solution is heated for aging, by the method (2) in which an alkali substance is added to active silicic acid obtained by subjecting an aqueous solution to the cation exchange processing and the mixture solution is heated for aging, by the method (3) in which alkoxy silane such as ethyl silicate is hydrolyzed and the obtained active silicic acid is heated for aging, or by the method (4) in which fine powder of silica is directly dispersed in an aqueous medium, generally colloidal silica particles having the diameter in the range from 4 to 1,000 nm (nanometers), and more preferably in the range from 7 to 500 nm are dispersed in an aqueous medium, and that the concentration of SiO2 is in the range from 0.5 to 50 weight % and more preferably in the range from 0.5 to 30 weight %. The document furthermore describes that forms of the silica particles includes a spherical form, an irregular form, a flat form, a plate-like form, a slender form, and a textile-like form.
Also in the case of the silica sol containing the nodular particles as described above, a high purity silica sol containing such impurities as Na or K as little as possible is required.
As a method of producing an aqueous silica sol with high purity, there is known a method using alkoxy silane containing few impurities as a starting material, and Japanese Patent Laid-Open Publication No. HEI 6-316407 (Patent document 7) discloses a method in which alkyl silicate is hydrolyzed in the presence of alkali and generated silicic acid is polymerized to obtain an aqueous silica sol. Also Japanese Patent Laid-Open Publication No. 2001-2411 (Patent document 8) and other documents disclose a method in which alkoxy silane is hydrolyzed in an acidic solvent and the obtained silicic acid monomer is polymerized in a basic solvent to obtain an aqueous silica sol. In the methods described above, there are the several problems, for instance, that stability of the obtained aqueous silica is not sufficient, or that an alkoxy group (OR group) remains, the density of obtained particles is low, and therefore a sufficient polishing speed can not be obtained when the silica sol is used as a polishing material.
In the method using water glass obtained by dissolving cullet obtained by solving glass sand with an alkali in water as a starting material, an aqueous silica sol can be obtained, for instance, by dealkylating the water glass and polymerizing the obtained silicic acid solution (containing silicic acid monomers much) in a basic solvent.
However, since many types of metals such as Al, Ti, Fe, Mg, and Ca are contained as impurities in the glass sand, alkali metals (represented by Na) remain much in the obtained aqueous solution in proportion to an amount of the metal ions, so that the aqueous solution is not suited for use as a polishing material for a semiconductor substrate.
Such metals as Al and Na can be reduced to some extend by processing the aqueous solution with an acid or a chelating agent, but the purity is not sufficient as a polishing material for a semiconductor substrate. Also high purity silica powder is used in place of the glass sand, but the high purity silica is expensive, and it take disadvantageously long time in solving the high purity silica with an alkali.
Japanese Patent Laid-Open Publication No. SHO 61-58810 (Patent document 9) discloses a method in which an aqueous solution of alkaline silicate having a prespecified concentration is contacted to a strong acidic cation exchanger, then an acid is added, the mixture solution is subjected to ultrafiltration and is furthermore contacted to a anion exchanger as well as to a cation exchange, and then ammonium is added therein to grow particles.
Japanese Patent Laid-Open Publication No. HEI 5-85718 (Patent document 10) discloses a method of producing an aqueous solution with high purity comprising the steps of: (1) decationizing a diluted aqueous solution of water glass to obtain an aqueous solution of active silicic acid, (2) processing the solution of active silicic acid with a strong acid, (3) deionizing the aqueous solution of active silicic acid having been processed with the strong acid to obtain a solution of active silicic acid with high purity, (4) adding an alkali to the aqueous solution to obtain a stabilized aqueous solution of active silicic acid, (5) evaporating and condensing this aqueous solution to grow particles and obtain a silica sol of the particles with the average particle diameter in the range from 10 to 30 mμ, and (6) contacting this silica sol to ion exchange resin to obtain an aqueous silica sol with high purity. In this method, impurities are removed (leaching) by adding a strong acid in the stage of active silicic acid, but there is a problem concerning stability of the active silicic acid after leaching, which easily causes gelatination of the sol.
Japanese Patent Laid-Open Publication No. HEI 6-16414 (Patent document 11) discloses a method of producing a silica sol with high purity comprising the steps of: adding a strong acid or a salt of a strong acid to alkali metal silicate or to an aqueous solution of active silicic acid to obtain a mixture solution, processing the mixture solution with ion exchange resin, adding the solution obtained in the process described above to the solution obtained by means of ion exchanging to prepare a silica sol, processing the obtained silica sol with ion exchange resin, and adding ammonia to the obtained silica sol. Also in this method, there are the same problems like those recognized in the invention disclosed in the Japanese Patent Laid-Open Publication No. HEI 5-85718.
In the method disclosed in Japanese Patent Laid-Open Publication No. 2003-89786 (Patent document 12), a chelate type ion exchange resin and a chelating agent are used as metal ion capturing agents. In this invention, it is require at first to contact an aqueous solution of alkali silicate to strong acidic ion exchange resin for removing alkali metals in the alkali silicate to prepare an aqueous solution of active silicic acid from the aqueous solution of alkali silicate, and then to contact the solution to chelate resin to removing multivalent metal ions. Namely in this invention, ion exchange is performed through multiple steps, so that the production efficiency is disadvantageously low. In addition, the chelate type ion exchange resin has strong selectivity to tervalent ions, and it is difficult to reuse the resin repeatedly, and when it is tried to reuse the chelate type ion exchange resin, a large amount of reclaiming agent is disadvantageously required.
When a chelating agent is used to capture metal ions, it is necessary to repeat cleaning with a ultrafiltration film for removing the remaining chelating agent and the chelating agent having capture metal ions, so that the production efficiency is rather low, and even when cleaning is repeated, it is difficult to completely remove the chelating agent, and the remaining chelating agent may give negative effects to the polishing performance.    Patent document 1: Japanese Patent Laid-Open Publication No. HEI 4-187512    Patent document 2: Japanese Patent Laid-Open Publication No. HEI 7-118008    Patent document 3: Japanese Patent Laid-Open Publication No. 2001-11433    Patent document 4: Japanese Patent Laid-Open Publication No. 2001-48520    Patent document 5: Japanese Patent Laid-Open Publication No. 2001-150334    Patent document 6: Japanese Patent Laid-Open Publication No. HEI 8-279480    Patent document 7: Japanese Patent Laid-Open Publication No. HEI 6-316407    Patent document 8: Japanese Patent Laid-Open Publication No. 2001-2411    Patent document 9: Japanese Patent Laid-Open Publication No. SHO 61-158810    Patent document 10: Japanese Patent Laid-Open Publication No. HEI 5-85718    Patent document 11: Japanese Patent Laid-Open Publication No. HEI 6-16414    Patent document 12: Japanese Patent Laid-Open Publication No. 2003-89786